Study on Share Rate of Support Structure for Super-Large Span Twin Tunnels with Small Interval
Abstract
:1. Introduction
2. Methodology
2.1. Daling Tunnel
2.2. Theoretical Load
2.3. Finite-Element Model
2.4. Field Measurement
- (1)
- Settlement of the vault of primary support was measured by total stations and steel rulers to find out the share rate of primary support.
- (2)
- Stress of the secondary lining (derived from the strain) was measured by concrete strain gauges. The gauges were put on both sides of the secondary lining at each point. This is for finding out the share rate of secondary lining. The arrangement of concrete strains is shown in Figure 6.
- (3)
- Total pressure of the primary support and secondary lining was measured by pressure cells arranged at the points A, B, and C at the outside of the primary support. This is for comparison and verification. The arrangement of pressure cells is shown in Figure 7.
3. Theoretical Calculation of Surrounding Rock Pressure
3.1. Failure Mode and Assumptions
- (1)
- Suppose that the ground was horizontal, the rock mass was homogenous, and isotropic and the twin tunnel was symmetry and parallel. Moreover, excavation of left and right holes is sequent and full section.
- (2)
- The excavation of the first hole is similar to an ordinary single-hole tunnel. It means that the fracture planes on both sides of the first hole, which are shown as A’C’ and M’J’ in Figure 8, are two inclined straight planes, and at an angle of to the horizon. In addition, the pressure inside and outside (shown in Figure 8) is symmetric.
- (3)
- When the following hole is excavated, the fracture plane at the outside of the hole (AC) is at angle to the horizon, while at the inside of the hole the angle of the fracture plane (MO) is assumed as . Focus on the triangle OJJ’, when the following hole is excavated, it is inclined to slide down along the plane JM. However, since the excavation of the first hole has induced a relative slippage at the plane OJ’ and undermines the cohesion force along the plane OJ’, usually the triangle OJJ’ will not slide and fracture along the plane JO. Instead, the tensile fracture plane will be formed in the triangle OJJ’, which is assumed as a vertical plane (OK). In summary, the fracture plane at the inside of the following hole is assumed as KOM.
- (4)
- According to the sliding trend of the triangle OJJ’ caused by the sequent excavation of the twin tunnel, and based on the theory of soil mechanics, the interactive force (N) in normal direction at the fracture plane OK must be less than the earth pressure at-rest. For safety, let N equal 0.
3.2. Calculation of Theoretical Load
4. Results and Discussion
4.1. Share Rate of the Primary Support
4.2. Share Rate of the Secondary Lining
4.3. Total Pressure for Verification
4.4. Discussion
- (1)
- The influence of the construction’s sequence of the first hole and following hole was not considered in this research, and the way of excavation was assumed as full section excavation.
- (2)
- This paper supposed that the stratum was consistent and the surrounding rock mass was ideal.
- (3)
- The surface of earth was presumed to be horizontal, and so on.
5. Conclusions
- (1)
- The formulas of calculating surrounding rock pressure of normal tunnels (usually including two or three lanes) in Chinese standard could be applied to the super-large span twin tunnels (including four or more lanes). The method was verified by field measurement.
- (2)
- The method of researching the share rate of the primary support and the secondary lining of the tunnel was proposed. The result demonstrates that the share rate of the primary support and the secondary lining of Daling Tunnel are both 40%. Usually for twin tunnels with super-large span in cities, not only the primary support but also the secondary lining should be strong enough to ensure the design is safe and reliable. In addition, this conclusion is conservative when compared to the measurement, which is practical for application in engineering.
- (3)
- The result of the theoretical calculation and numerical simulation was compared with the measurement to evaluate the research methodology and achievements. These two results match properly, which verify the correctness of this study. Hence the conclusions and the method of this research can make some reference to the design, construction, and maintenance of super-large span twin tunnels.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Parameter | Value |
---|---|
Unit weight, (kN/m3) | 20 |
Simplified internal friction angle, | 45° |
Real internal friction angle, | 22° |
Friction angle of the sliding plane, | 13.2° () |
Span, B (m) | 19.4 |
Height, Hl (m) | 12.9 |
Net interval, D (m) | 12.6 |
Buried depth, H (m) | 20 |
Part | Young’s Model /GPa | Poisson’s Ratio | Unit Weight /kN/m3 | Thickness /m |
---|---|---|---|---|
Primary Support | 25.3 | 0.2 | 25 | 0.3 |
Secondary Lining | 28 | 0.2 | 25 | 0.7 |
Item | Position | Axis Force /kN | Moment /kN·m | Eccentricity /mm | Attribute | Safety Factor |
---|---|---|---|---|---|---|
Simulation | Vault | −1080 | −222 | 493 | All is small eccentricity | 9.47 |
Left haunch | −1432 | 204 | 430 | 8.19 | ||
Left sidewall | −1572 | −32.6 | 308 | 10.41 | ||
Right haunch | −1432 | 204 | 430 | 8.19 | ||
Right sidewall | −1572 | −32.6 | 308 | 10.41 | ||
Measurement | Vault | −1723 | −53 | 318 | 7.85 | |
Left haunch | −1216 | 24 | 307 | 11.54 | ||
Left sidewall | −781 | 71 | 378 | 14.0 | ||
Right haunch | −1348 | −36 | 314 | 10.17 | ||
Right sidewall | −862 | 38 | 332 | 15.00 |
Measured Point | Theoretical Load /kPa | 80% of the Theoretical Load /kPa | Measured Pressure /kPa | Deviation /% |
---|---|---|---|---|
A | 286.90 | 229.56 | 180.94 | 21.18 |
B | 211.63 | 169.30 | 142.68 | 15.73 |
C | 98.40 | 78.72 | 65.36 | 16.97 |
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Zhao, X.; Sun, K.; Zhen, Y.; Hong, Y.; Zhou, H. Study on Share Rate of Support Structure for Super-Large Span Twin Tunnels with Small Interval. Appl. Sci. 2022, 12, 7498. https://doi.org/10.3390/app12157498
Zhao X, Sun K, Zhen Y, Hong Y, Zhou H. Study on Share Rate of Support Structure for Super-Large Span Twin Tunnels with Small Interval. Applied Sciences. 2022; 12(15):7498. https://doi.org/10.3390/app12157498
Chicago/Turabian StyleZhao, Xuwei, Keguo Sun, Yingzhou Zhen, Yiqin Hong, and Huichao Zhou. 2022. "Study on Share Rate of Support Structure for Super-Large Span Twin Tunnels with Small Interval" Applied Sciences 12, no. 15: 7498. https://doi.org/10.3390/app12157498